Polyaniline (PANI) is the polymer formed by chemical or electrochemical oxidative polymerization of aniline. Polyaniline can vary in its degree of oxidation and protonation depending upon the conditions of the polymerization and the presence of a source of acidic protons. In its partially oxidized and protonated form called emeraldine salt, it is electrically-conductive and thus has attracted great attention for several decades to determine the basic physics of its electrical conductivity and to realize its potential in numerous applications ranging from chemical and bio-sensors, electrochromic windows, light emitting diodes, batteries, supercapacitors, photovoltaics, anti-corrosion coatings, and even artificial muscles. Recent literature highlight the unique physical character of coatings employing polyaniline and its various composites with graphene, carbon nanotubes, and a wide variety of nano-particles and nano-structures.
A film of polyaniline can be readily formed by oxidation at an anode surface during electropolymerization. This polymerization process has the limitation of requiring a conductive substrate and would likely be difficult for flexible roll-to-roll coating and manufacturing operations. Typical chemical oxidative polymerization of polyaniline produces a powder that is insoluble in water and sparingly soluble in solvents such as N-methyl pyrrolidone if crosslinking through the aniline monomer ortho-position is limited in the polymerization process. To be electrically-conductive, this PANI powder must be not over-oxidized and must be partially protonated or doped with a strong acid such as hydrochloric acid or a sulfonic acid such as p-toluene sulfonic acid or camphor sulfonic acid that will not volatilize from within a thin coating. Such polymerization can also be carried out in the presence of an acidic polymer such as poly(styrene sulfonic acid) that forms a polymer complex providing aqueous solubility and a permanent acidic dopant for conductivity similar to the conductive polymer composite of poly(3,4-ethylenedioxythiophene)(PEDOT) and poly(styrene sulfonic acid)(PSS) typically referred to in the art as PEDOT:PSS. These composites of PSS and PANI can then be coated from water, but a method to pattern and insolubilize the coating is still required to make it compatible with and durable in further processing steps that may be required to form an article having an electrically-conductive pattern.
Many uses of PANI require a patterning process to form electrodes, interconnects, or sensing patterns designed from the electrically-conductive polymer. Printing methods such as screen, flexographic, gravure, and inkjet are possible and have been discussed in the literature. Such PANI printed images will still be soluble in some solvents used during printing and may exhibit problems with durability or compatibility with other materials required for a making such devices. Patterning of polyaniline films has been carried out using traditional lithographic methods where a soluble polyaniline coating is masked by an imaging polymer and the unmasked PANI is dissolved. The masking polymer may then be removed if necessary if the underlying PANI is not soluble in the solvent for the mask polymer. Vacuum deposition and etching methods well known in the semiconductor industry could also be used to form coatings and patterns, but the process is expensive and does not lend itself to simple roll-to-roll coatings operations.
U.S. Pat. No. 6,045,977 (Chandross et al.) describes a method for making a device in which a conductive PANI salt layer is formed on a substrate and patterned into a desired configuration. U.S. Pat. No. 8,932,494 (Liao et al.) describes a method for forming electrically-conductive PANI-based composites by photo-irradiation a base form of PANI and a photo acid generator that is sensitive to the irradiation, converting the non-conducting base form of PANI to conductive PANI salt in a polymer composite.
While it is possible to form coatings of PANI from organic solvents or even from water if the PANI is complexed with a strongly acidic polymer like PSS, the patterning process can create substantial difficulty, expense, and complexity in building functional devices with electrically-conductive patterns. Furthermore, without some curing or crosslinking process, these electrically-conductive patterns will still be soluble in water or other solvents that may be used to build the device. For example, often polyaniline requires post treatment with acidic solutions or oxidizing or reducing agents to obtain the desired electrical conductivity. A simple method of forming a water-insoluble and durable coating or pattern of polyaniline that is permanently doped in its conductive emeraldine salt form without further processing could provide simplified and lower cost methods to form devices with patterns of polyaniline.
There remains a need for a simple method for producing electrically-conductive polyaniline in the form of uniform layers or patterns on various substrates including continuous flexible polymeric webs in roll-to-roll manufacturing operations, which electrically-conductive layers or patterns can be used to provide various devices including display touch screens.